The present invention provides a treatment for aluminides which makes the aluminides better suited for use in high-temperature environments.
As a means of saving weight, the blades of gas turbines can be made of aluminides. The incentive to save weight in aircraft turbines is obvious. Even in stationary ground-based turbines, there is an incentive to make the blades of aluminides. The turbine rotates on bearings which do wear eventually. After the bearings begin to wear, the mass of the rotor begins to precess, which further accelerates the wear of the bearings.
The weight saving due to the use of aluminides is considerable; the density of titanium aluminides is only about half the density of the superalloys used for turbine blades.
A gas turbine engine comprises a compressor and a turbine. In the compressor, the air for combustion is compressed to the pressure at which the fuel is injected and combustion takes place. In advanced turbines, the compression ratio is as high as 30, and the temperature of the compressed air entering the combustor reaches about 800.degree. K. or about 530.degree. C. Oxidation is not a serious problem at this temperature, so it is not essential that the surface of the compressor blade be particularly resistant to oxidation.
The temperature of the combustion gas entering the turbine can reach 1300.degree. C. in advanced turbines. This temperature is much too high for aluminides, even when treated to make them resistant to oxidation. Turbine exhaust temperatures are much lower. The turbine exhaust temperature is about 680.degree. C. for the Concorde in supersonic flight and as low as 500.degree. C. for stationary turbines. Thus, it is possible to use aluminide blades in the later stages of the turbine.
The present invention provides a treatment for the aluminide blades which makes it possible to introduce such blades into the hotter stages of the turbine.